Scales Aren’t Just for Weighing…Check Your Plants for Scales

— Written By Bill Hanlin and last updated by JoAnne Gryder

I have had three homeowners bring in samples this week that were dead or dying shrub branches. Some were easy to diagnose while it was not obvious on one sample what was causing the problem. As it turned out, all three had scale infestations and, if left untreated, would eventually kill the shrub.

Scales are small insects that settle down and feed on the stems and leaves of certain plants. Once the scale starts feeding, it forms a protective covering that protects it from predators and most insecticides. This insect never moves again once it settles down to feed.

There are two broad categories of scales that include soft and armored scales. Soft scales secrete a waxy covering and are usually larger than armored scales. As the name implies, armored scales secrete a flat-shield type covering and are more common pests in our area.

Soft scales also produce a substance called honeydew which can lead to a black honeydewfungus called sooty mold to grow on the leaves. Armored scales do not produce honeydew.

Scales feed on a specific plant type or on a narrow range of plant species. Some of the most common scales in our area include euonymus, white peach, San Jose, camellia and oyster shell scales. If you want to avoid scale infestations, then you may not want to plant trees and shrubs that are prone to scale infestations.

As mentioned earlier, it is very difficult to control scales by spraying an insecticide directly on them. Horticultural oils are effective since they work by suffocating the insect. In order for oils to work the plant must be thoroughly coated where the infestation is occurring.

Some soil applied, systemic insecticides are effective in controlling scales. Products containing the active ingredient imidacloprid can control soft scale infestations but not armored scales. Insecticides containing dinotefuran are effective in controlling armored scales. Please read the label of the products very carefully as both insecticides are very toxic to pollinators like honeybees.